This present disclosure relates to an offshore lifting crane. More specifically the disclosure relates to an offshore lifting crane comprising a support structure, a crane boom connected to said support structure, a winch drum rotatable around a longitudinal axis thereof, a winch drum drive means and an elongated hoisting member having a first end connected to said winch and a second end connectable to a load, said elongated hoisting member extending over at least a part of said crane boom.
Offshore lifting cranes and their related equipment are getting increasingly large and heavy in order to keep up with the requirements for lifting continually heavier loads often in increasingly deep waters. Lifting cranes for deep water operations need winch drums suitable for storing several thousand meters of wire rope, often in the order of 3000 meters or more, thus requiring large, heavy winch drums with equally large footprints. For hoisting loads in deep water operations it is often desirable to use fibre ropes due to their reduced weight compared to traditional steel wire ropes. However, fibre ropes stored on winch drums operating in heave compensation mode and accommodating more than two layers of fibre rope are subject to unacceptable wear, leading to an unacceptable short lifetime of the fibre rope. The excessive wear results mainly from a reduced radial stiffness of the fibre rope due to heating resulting from friction in repeated bending cycles in heave compensation.
As an example, a 250 short tons crane dimensioned for subsea hoisting operations down to 3000 meters, using fibre rope stored in no more than two layers on a winch drum with a diameter of 6 metres, will require the winch drum to be approximately 8 meters long. It goes without saying that such a long winch drum would be very unpractical to place on a vessel, either on a crane or below deck. As such, it is currently not feasible to use fibre ropes on large actively heave-compensated offshore subsea crane-/winch systems. Further, a winch drum and its drive means are often placed at an upper portion of an offshore crane, typically on a platform above the crane housing, the platform extending horizontally away from the crane housing in the opposite direction of the crane boom, thus giving the crane a tail swing and a relatively high centre of gravity. For very large crane-winch systems this has been solved by providing the winch below deck of the vessel on which the lifting crane is installed. This however, has the drawback of significantly complicating the installation, thus increasing installation time and cost.
Thus, the present disclosure provides an offshore lifting crane suitable for deepwater hoisting operations with heavy loads, even in heave compensation, where the footprint of the winch an winch drum, and preferably also the overall weight of the crane-winch system, is reduced. The present disclosure offers an easy installation of the offshore crane on a vessel so as to reduce installation time and cost.
FR 2463093 discloses a compact lifting crane with a winch drum and a spooling device included inside the mast of the crane.
WO 2014/025253 A1 discloses crane vessel and a method for lowering an object from such a vessel into the sea.